16 1. Various Ways of Representing Surfaces and Examples Figure 1.12. From the sphere to a disc via geographic coordinates. disc, where the actual distance between points is much less than the Euclidean distance (since every point on the boundary is identified)— notice how much Antarctica is stretched out in Figure 1.12. This gives us our first example of a Riemannian metric (which for the time be- ing we may simply think of as a notion of distance) on D2, apart from the usual Euclidean one. Exercise 1.7. Stereographic projections from the north and south poles introduce two coordinate systems on the sphere minus the poles. Find the coordinate transformation from one of those systems to the other—that is, if a point on the sphere has coordinates (x, y) in the coordinate system projected from the north pole and (x , y ) in the projection from the south, find (x , y ) as a function of (x, y). c. Parametric representations. While the idea of putting local coordinates on a surface will turn out to be more useful in general, we will occasionally have reason to deal with parametric representations. There are two important distinctions between these two methods of introducing coordinates on a surface. First, local coordinates involve a map from the surface to a plane domain, while a parametric representation is a map from a plane domain to the surface. Formally, then, these two constructions are mutual inverses. The second distinction is that a local coordinate system usually does not attempt to cover the entire surface by a single coordinate
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